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Author Topic: Assesing voltage drop in high current connections  (Read 665 times)
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I am trying to assess the hack job done on this robot.  Most of the high current lines (40-160 amps) are undersized (10ga) and very poorly crimped.  I found a large loss in power along the lines, which I am fixing now.  However I expected the main losses to be in the crimp connections, but from the small amount of re-working I have done, it seems that even in these short (20-60 inch) runs, it is the conductor which has more  effect than the connector, even tho the math says the resistance in the wire is much higher than the speced ohm/foot.

My question is that I am looking for a rule of thumb.  What defines "good enough".  yes the robot works, but initially the line loss was 25% the total voltage.  If there is no rule of thumb, let me give you my personal scenario.

12 volt supply
18 feet of wire 10 ga stranded (I can cull this massivly to <10 i hope, and use the remains to double up)
~25 amp average load, breakers pop if I hit 160, which I never have, even with the old 24 volt supply

I expect to get the drop down from 2.82@48 to under 2.  with my current wiring runs the theoretical minimum is about 1.1 at 48 amps.  Sadly the 2.82 number is accurate, but the 48 is a rough estimate as I don't currently have an ammeter rated past 10A, but I will by this week and will do more rigorus testing.

Anyways, I don't want to waste my time trying to get the numbers lower than is practically achievable

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You could try additional soldering of the crimped connections. You can get an inexpensive IR temperture gun and look for wiring, connections, etc., that are higher in temperature than ambient.
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Yeah and under $10.00 too I think, doesn't need to be accurate, just be able to sense things slightly above the ambient. Deg C or F are unimportant just the lowest possible for best efficiency, Watts that heat wires... Don't Drive Tires...
I'm Sorry, I couldn't resist that.

Doc
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Crimped connections can be very good and are preferred to soldered connections in many situations. However, to make a good low resistance crimp connection, you need one of the expensive crimp tools with a ratchet and gears, not the cheap sort sold in DIY stores.
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I have a nice paladin ratchet crimp connecter and will be making all new connections.  None of the wires get hot, I used the grab test all day while measuring voltage drop.  There is nothing "wrong" with the current setup, I am more trying to figure out what is generally accepted as "good enough" in these situations.

hell the only hot connection I found was on one of the auto-reset breakers cause the nut had backed off.  is 25% wiring eff good?  I assume not, or maybe thats why everyone loves HV power supplies.  I know HOW to fix the problems, I am looking more for a definition of fixed, ya dig?
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also as a predominantly automotive guy, I avoid solder connections on things that vibrate, but that is a whole nother topic, I don't want to sidetrack this thread.
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There is nothing "wrong" with the current setup, I am more trying to figure out what is generally accepted as "good enough" in these situations.

That is going to be up to you. if you want lower resistance in the wiring, use larger solid copper wires, or skip the copper and use large pure silver wires/busbars. It is your situation, so you are going to have to make the decision on what is good enough for you.
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Does the drive gear ratio allow for minimum motor current. Just a random thought...

Doc
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There is nothing "wrong" with the current setup, I am more trying to figure out what is generally accepted as "good enough" in these situations.

That is going to be up to you. if you want lower resistance in the wiring, use larger solid copper wires, or skip the copper and use large pure silver wires/busbars. It is your situation, so you are going to have to make the decision on what is good enough for you.

Silver wire of the same conductivity as copper would be 11% heavier and cost 130 times as much, whereas aluminium wires of the same conductivity would be half the weight and about 40% of the cost of the copper.  Aluminium is the best choice for heavy current wiring and is used heavily in industry for this reason.

Aluminium would take up 70% more volume though.

For the ultimate a cryogenic system and superconducting cable would give zero losses (except for the power needed to chill the liquid helium!)
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